Embodiments of the present invention relate generally to methods and devices for depositing viscous materials onto a printed wiring board. In one aspect, the present invention relates to methods and devices for compressing viscous materials, such as solder paste, through openings in a perforated substrate, such as a patterned screen or stencil.
Surface Mount Technology (SMT) involves placing circuit components onto circuit paths embedded on the upper surface of a printed wiring board and then soldering the components in place by a process called xe2x80x9creflow solderingxe2x80x9d. Before the circuit component is placed on the printed wiring board, however, it is desirable to apply solder paste to the area on the printed wiring board where the component is to be soldered into place.
Conventional methods do exist to deposit (xe2x80x9cprintxe2x80x9d) solder paste onto desired areas of a printed wiring board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the printed wiring board.
U.S. Pat. No. 4,622,239 describes such a method and device for dispensing viscous materials. The method includes forcing a viscous material from a housing through an opening and depositing it onto a stencil between a pair of flexible members (parallel squeegee blades) which depend from the housing on either side of the opening and are in contact with the stencil. The ends of the flexible members are not connected and remain open ended. The viscous material, accordingly, is not contained within an enclosed area when it is deposited on the surface of the stencil. Movement of the housing and the flexible members horizontally across the stencil causes the trailing flexible member to force the viscous material through the openings in the stencil. U.S. Pat. No. 4,720,402 describes a similar method and device except that the leading flexible member is raised off of the stencil during movement of the housing.
U.S. Pat. Nos. 5,133,120 and 5,191,709 describe methods for filling through-holes of a printed wiring board via a mask with pressurized conductive filler material by means of a nozzle assembly unit having a nozzle tip member. The nozzle tip member, however, is designed only to dispense the pressurized conductive filler material through the mask to a single through-hole. The nozzle tip member then xe2x80x9cscansxe2x80x9d the printed wiring board for a second through-hole to fill. The nozzle tip member has a blunt end section which rests on the mask and a circular exit, the diameter of which may be increased or decreased by changing the nozzle tip member. The nozzle tip member dispenses the filler material without controlling unwanted flow of xe2x80x9cexcessivexe2x80x9d filler material back through the stencil. Additionally, the nozzle tip member does not define a contained environment where xe2x80x9ccompressionxe2x80x9d of the filler material takes place through the mask followed by the immediate shearing off of the filler material within that contained environment from the surface of the stencil. In fact, the nozzle tip member itself provides no effective means for shearing off filler material from the top of the stencil, rather, after the through hole is filled and filler material xe2x80x9cbacks upxe2x80x9d through the stencil, the nozzle tip member moves forward whereupon the xe2x80x9cexcessivexe2x80x9d filler material is then wiped off by a separate, single, flexible squeegee member which is designed for unidirectional use only.
Unfortunately, these conventional efforts do not provide a contained environment for compression of viscous material through holes in a stencil and shearing of viscous material within the contained environment from the upper surface of the stencil. Reliance upon squeegee movement to force the viscous material, such as solder paste, through the stencil openings can lead to damage and eventual failure of both the squeegee blades and the stencil due to repeated friction. Since conventional efforts do not provide a contained environment in which compression and shearing is accomplished, waste of the viscous material is frequently encountered.
Conventional efforts, therefore, (1) fail to maximize the efficiency of printing solder paste onto a desired area of a printed wiring board and (2) fail to minimize waste of the solder paste during the printing process. A need therefore exists to develop a method for printing solder paste onto a printed wiring board and a device suitable for use therewith which overcomes the deficiencies of the conventional efforts.
Other methods and assemblies utilize a pneumatically driven piston assembly which typically resides upon the compression head and which typically includes a container having a certain amount of viscous material and at least one piston which is selectively and movably deployed within the container and which is effective to cause a certain amount of the contained viscous material to be selectively dispensed and communicated to the compression head.
These prior assemblies and methodologies suffer from at least several drawbacks. First, the piston-viscous material containment assemblies are relatively heavy and bulky, thereby requiring undesirable and relatively costly structural or xe2x80x9csupport-typexe2x80x9d modifications be made to the existing compression head assembly. These pneumatically driven piston assemblies are also relatively difficult to service and undesirably interrupt the printing process each time that maintenance is performed upon them or additional viscous material is added to the container. Moreover, these prior piston-viscous material containment assemblies usually inject or communicate at least a portion of the contained viscous material into a top portion of the head. The communicated viscous material is typically made to traverse through the compression head and is made to exit at or through a bottom portion or viscous material dispensation aperture which is typically and integrally formed within the compression head.
This arrangement typically causes the velocity of the emitted or exiting viscous material to be non-uniform and to have a substantially non-uniform velocity profile. That is, the velocity of the viscous material which exits the aperture and which enters the stencil perforations is non-uniform and varying. The exiting viscous material therefore provides or has a non-uniform velocity profile. Moreover, that portion of the dispensed viscous material, which encounters a solid or non-perforated portion of the stencil, such as a xe2x80x9cpaste-filledxe2x80x9d hole, causes a certain amount of xe2x80x9cbackpressurexe2x80x9d to be created within the compression head and/or within or through the dispensation aperture. The created xe2x80x9cbackpressurexe2x80x9d is substantially non-uniform and varies from location to location within the compression head and varies from location to location within or through the viscous material dispensation aperture. The selectively dispensed viscous material thereby provides or has a non-uniform pressure profile.
Particularly, most of the received viscous material flows through a path or paths which is (are) substantially and axially aligned with the material reception aperture or that location through which the viscous material is communicated into the compression head. This flow pattern is due to the fact that this path or (these paths) has (have) a relatively lower flow resistance than other flow paths which are not substantially aligned with (are remotely positioned from) the material reception aperture. Accordingly, material traversing this path (or paths) exits the compression head at a substantially higher velocity than the material flowing through the various other paths. It is this dissimilarity in flow resistance which causes the viscous material to be dispensed into the stencil perforations at velocities which vary from xe2x80x9clocation to locationxe2x80x9d within the material dispensation aperture, thereby forming a substantially non-uniform velocity dispensation profile (e.g., the xe2x80x9cvelocity dispensation profilexe2x80x9d or xe2x80x9cvelocity profilexe2x80x9d comprises the velocity values associated with the viscous material which is communicated to the stencil perforations through various locations within the material dispensation aperture). Similarly, this prior arrangement further causes the creation of non-uniform pressure profiles (e.g., the pressure xe2x80x9cprofilexe2x80x9d comprises the values of the xe2x80x9cbackpressurexe2x80x9d, at various locations within the material dispensation aperture, which is created by the viscous material). These non-uniform pressure and velocity profiles cause different amounts of viscous material to be placed upon the various portions of the stencil surface. This non-uniform material distribution thereby causes the creation of unacceptably poor quality circuit boards and further causes a waste of the viscous material.
The present invention is directed to overcoming some or all of the previously delineated drawbacks while allowing for the selective emission or dispensation of viscous material having a substantially uniform pressure and velocity profile, while reducing the amount of printing and/or circuit creation interruptions during the circuit board printing process, and providing a compression head assembly which may be easily serviced and maintained.
The present invention includes a novel apparatus and method for dispensing viscous material through openings in a stencil. Embodiments of the present invention include a process herein referred to as xe2x80x9ccompression printingxe2x80x9d wherein pressure is applied to a viscous material within a contained environment defined by a compression head cap so as to compress it through openings in a stencil.
The apparatus of the present invention includes a reservoir containing viscous material which is operably connected to a pressure source. The reservoir is in fluid communication with a housing which terminates in a substantially uniform opening defined by a compression head cap formed from contiguous walls. During operation of the apparatus, the compression head cap is placed in contact with a stencil having a plurality of openings therein. The compression head cap and the stencil form a contained environment. The pressure source then applies pressure against the viscous material contained in the reservoir forcing it from the reservoir into the housing and to the compression head cap. The contiguous walls of the compression head cap act to contain and to direct flow of the pressurized viscous material to the top surface of the stencil and then through the openings in the stencil.
It is accordingly an object of the present invention to provide a novel apparatus for compressing a viscous material through openings in a stencil by means of a pressure source. It is a further object of the present invention to increase the efficiency of printing viscous material onto a desired area of a printed wiring board and to minimize waste of the viscous material during the printing process.
According to one aspect of the invention a viscous material dispensing apparatus is provided. The dispensing apparatus includes a compression head which selectively receives and dispenses viscous material. The compression head has a dispensation aperture through which the viscous material is selectively dispensed, a viscous material reception aperture through which the viscous material is selectively received, and a selectively expandable diaphragm which causes the received viscous material to dispensably exit the compression head through the dispensation aperture.
According to yet another aspect of the invention a viscous material dispensing assembly is provided. The assembly includes a container of viscous material, the container having an aperture which allows the contained viscous material to selectively exit the container, and a compression head having a top portion which includes a selectively expandable diaphragm, a side portion which communicates with the aperture of the container and which allows the viscous material to flow into the compression head in a first direction, and a bottom portion which includes a material dispensation aperture, the diaphragm being selectively movable in a second direction which is substantially perpendicular to the first direction and being effective to selectively cause the received viscous material to dispensably exit the compression head through the dispensation aperture.
According to a further aspect of the invention a method of dispensing viscous material is provided. Particularly, the method includes the steps of providing a head having a dispensation aperture, communicating the viscous material to the head, providing a selectively expandable diaphragm within the head, and selectively expanding the diaphragm, thereby causing the received viscous material to be selectively dispensed through the dispensation aperture.
Other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.